You run grep -E '(foo|bar)+' file.txt but get an error: grep: invalid regular expression. What is the likely cause?

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Linux CLI - Text Processing

You run grep -E '(foo|bar)+' file.txt but get an error: grep: invalid regular expression. What is the likely cause?

AYou forgot to quote the pattern

BThe parentheses are not escaped and your grep version does not support unescaped ()

CThe + symbol is not valid in extended regex

DThe file.txt does not exist

Step-by-Step Solution

Solution:

Step 1: Check regex syntax and grep version
Some grep versions require escaping parentheses even with -E, so unescaped () cause invalid regex error.
Step 2: Verify other options
Quoting is done, + is valid in extended regex, and file existence error would be different.
Final Answer:
The parentheses are not escaped and your grep version does not support unescaped () -> Option B
Quick Check:
Escape () if grep -E errors [OK]

Quick Trick: Escape parentheses if grep -E gives regex error [OK]

Common Mistakes:

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